Fluxless Soldering in Wave Soldering Equipment Using Forming Gas

G. L. Arslanian Air Products and Chemicals, Inc. U.S.A.

1 Abstract Econopack I SMT was used to evaluate the forming gas soldering process. The wave soldering machine A fluxless method has been developed using forming gas is equipped with a long inerting tunnel, two bottom IR to reduce metal oxides and enhance wetting of solder on preheaters and an in house fabrication of a contour printed circuit board metal interconnects. Dilute mix- with a wave type "A" (see Figure 1 and 2). tures of hydrogen in an inert nitrogen carrier gas were used to solder components to printed circuit boards with Sn/Pb To perform forming gas fluxless soldering, three require- 63-37 tin/lead coating and copper metallizations. The form- ments must be met: ing gas, a reactive gas, reduced the surface metal oxides 1. The forming gas must reduce the critical surface sufficiently to allow for wetting and to form metallurgical oxides in a vaporized form at soldering tem- solder joints. perature (260 degree Celsius). Introduction 2. The forming gas must react quickly, on the or- Solder joints are routinely used to form the interconnec- der of a few seconds to provide sufficient tions between the components and printed circuit boards manufacturing throughput. (PCB). These joints are made between solderable metal- 3. The forming gas must be reactive enough to re- lized surfaces such as Cu, Ni, or tin/lead. The metallized duce metal surface oxides but not cause a detri- layers are often exposed to an oxidizing environment mental effect on the soldered assembly. (room temperature in air or post reflow operation) for an extended period of time prior to wave soldering. An oxi- The premixture (high pressure cylinders) of hydrogen and dized metal surface inhibits solderability. Once the oxides nitrogen is gaseous at room temperature and can be eas- are removed, the solder flows over to the metallization and ily introduced into the wave soldering equipment for the forms a metallurgical solder joint. A clean non-oxidized solderability tests. surface has a higher surface energy than an oxidized one and hence a liquid will more readily wet to a clean sur- The concentration of the hydrogen in the inert gas carrier (nitrogen) is on the order of 3 percent. As mentioned face. above the reduction rate of oxides is more efficient in the Forming gas (3% hydrogen, balance nitrogen) fluxless presence of pure hydrogen. Due to safety concerns and soldering is a process in which liquid flux is replaced by a since the wave solder system was not equipped to proc- fluxless process in which hydrogen is added to an inert ess 100% hydrogen, the mixture of 3% hydrogen was carrier gas (nitrogen). By definition, the forming gas is a chosen for this test program since this concentration is flux or reactive species in that its function is to reduce sur- below the flammability limits of hydrogen in air. face metal oxides in order to enhance solderability. The reactive nature of hydrogen is well documented in the lit- Solderability tests with 63SN - 37Pb solder were per- erature. Reduction of solder oxides by hydrogen in form- formed with the wave soldering equipment to explore the ing gas has been observed in other microelectronics appli- effect of wettability and solder joint formation for a variety cations.[1, 2] Experimental results indicate that an initiation of forming gas process variations. The solder pot temperature was at 260 degrees Celsius. The metal intercon- temperature exists for hydrogen to reduce solder oxides, below which the reduction is insignificant and above which nects tested were copper and tin/lead. The test parame- the reduction process is accelerated. This initiation tem- ters and results are shown in Table 1. perature is found to be dependent on the area of solder to be reduced or the volume of oxide present. The initiation temperature is about 319°C for Pb oxide and 400°C for Sn Results oxide. The isothermal reduction rate is found to be a func- At a soldering temperature of 260 degree Celsius and the tion of forming gas concentration. The reduction rate is forming gas injected through the contour diffusers, the directly proportional to the hydrogen concentration in the forming gas reacts with the metal oxides but not enough [3, 4, 5] forming gas, consistent with a theoretical model. This to form an acceptable solder fillet on the top side of the model predicts that the average reduction rate at 370°C in board. Research has shown that SnO2 is reduced to SnO pure hydrogen is about 3.3 nm per minute, in terms of de- in an hydrogen atmosphere at temperatures starting at crease in oxide thickness. Another plus in using forming 200°C.[3, 4] SnO has greater wettability characteristics gas as a fluxing agent is it leaves no residue therefore no than SnO2. The forming gas injected in the second pre- post solder cleaning is required. heater at higher temperature (537 degree Celsius), reacts The experimental techniques and solderability results dis- rapidly with the metal oxides and, consequently, results in cussed here show that forming gas soldering as a replace- suitable soldering quality (top and bottom solder fillets). ment for liquid flux has promise as a production process. The temperature in the preheat zone is above the activa- tion temperature for hydrogen to reduce the metal oxides. The temperature of the top of the board was about 115 degree Celsius before contact with the solder wave. The Experimental Procedure following is a detailed discussion of the test results. The fluxless soldering process trials used the following To determine baseline characteristics of the printed circuit standard production equipment and modifications: boards with the Value solder mask, the testing of Group A An Electrovert wave soldering machine, model was performed using forming gas introduced into the

2 hood enclosing the solder pot (pre and post wave) and with the same set-up. The forming gas can be a suitable injecting nitrogen in the contour shroud (Figures 1 and 2). option if it is properly diffused in the preheater zones and An acceptable bottom side fillet was observed with minimal in the contour zone for fluxless soldering in wave soldering second side (top) wetting (Figure 3). Forming gas was in- equipment. Modifications of the diffuser locations within troduced into Preheat Zone No. 1 via a 0.250" OD the preheater area and possible additional diffuser assem- stainless steel tube (Group B). The temperature of Pre- blies will allow for a more accurate introduction of the heat Zone No. 1 was set at 1,050°F (565°C). There was forming gas into the equipment. A separate flow panel can no wetting to the second side (Figure 4). For Group C be used to allow individual control of the forming gas flow the nitrogen was shut off to the contour shroud. The sol- rates in the different areas and for ease in switch-over be- dering results were similar to the baseline run with im- tween reactive gas (forming gas) and nitrogen inerting proved wetting to the second side (Figure 5). The wetting blanket for the wave area. The forming gas also acts as a observed was probably due to the temperature of the blanket around the wave area that displaces oxygen and preheat atmosphere which was above the initiation point prevents oxidation during soldering. to reduce SnO to Sn. 2 A second family of boards with the CIBA 52 solder mask Conclusion were processed. These boards were processed in nitrogen only to determine baseline characteristics and demon- Forming gas can be used in a fluxless soldering process strated fair bottom side wetting but no second side wet- to reduce metal oxides of copper and tin/lead and to facili- ting. A series of baseline runs were completed that had tate the formation of high quality solder joints if it is prop- forming gas introduced into the hood area as in Group C erly diffused in the preheater zones and the contour zone with no nitrogen flow to the contour shroud. Wetting was of a wave soldering machine. The use of elevated prehea- acceptable on the bottom side; however, no second side ter temperatures (535 degree Celsius) results in rapid re- wetting was observed. Injection of forming gas only into duction of the metal oxides. Preheat Zone No. 1 with nitrogen flowing to the other areas gave reduced wetting to the bottom side with no sec- The forming gas leaves no residue after soldering. Equip- ond side wetting. To promote wetting on the second side ment modifications, including additional diffusers to permit a more consistent flow of forming gas into the preheat of the board forming gas was introduced in the diffuser system to the contour. This change in the injection pattern zones and contour zone and separate flow controls for the of the forming gas gave a minimal improvement to the bot- forming gas, are planned and develop an optimized atmosphere system for production use. Although initial trials tom side wetting but did not promote second side wetting. The improved wetting on the bottom side may be attribut- showed variation and non-optimized results, the positive able to the possible reduction of SnO2 to SnO. The tem- results in oxide reduction capability demand further test- perature of the atmosphere with the solder pot area is ing. Additional trials will be run to expand on the initial re- greater than 200°C and with forming gas diffusing in this sults obtained in this study. area, some reduction probably occurred.

In an effort to maximize reduction of SnO2, the injection points for the forming gas were changed from the above trials. For Group D the forming gas was introduced into Preheat Zone No. 2 and the diffuser system of the contour shroud. The printed circuit board demonstrated good bottom side fillet with 100% side hole wetting and good second side wetting (Figure 6). With the introduction of forming gas into the Preheat Zone No. 2 at a temperature of 563°C the reactivity time was increased due to the extended exposure in an hydrogen atmosphere. The forming gas in the wave area could have possibly contributed in two ways. Initially the forming gas diffused into the Pre- heat Zone No. 1 to maintain the reduced state of the tin and secondarily in the wave area to reduce SnO2 to SnO or to maintain the oxide as SnO. One final experiment was run that involved bare copper boards with organic solder preservative (OSP) coating. Forming gas was introduced in the hood area (pre and post wave) and through the contour shroud diffusers. Ni- trogen was injected into Preheat Zone 1. Mixed results were observed. Group E demonstrated good bottom fillets with 100% side hole wetting and fair second side wetting to the pads (Figure 7). The forming gas was not properly diffused in the preheater zones and variations were observed from board to board

3 Acknowledgments The authors gratefully acknowledge Bruce Adams, Ralph Richardson and Christine C. Dong of Air Products and

Chemicals, Inc. for their support and technical discus- sions, and Jacques Bechard and Mark Legros of Air Prod- ucts, Canada for providing technical assistance during the experimental runs.

References 1. R.D. Deshmukh, M.F. Brady, R.A. Roll, and L.A. King, "Active Atmosphere Solder Self-Alignment and Bonding of Optical Components," The International Journal of Microcircuits and Electronic Packaging, [2] 16 , pp. 97-107 (1993). 2. Rao Bonda and Kenneth Kaskoun, "Flip Chip As- sembly of 34K Thunderbolt Die on Glass Substrate for the THUNDER Build," Proceedings of the 1995 Summer Motorola AMT Symposium, pp. 269-276, July 1995. 3. W.A. Oates and D.D. Todd, "Kinetics of the Reduc- tion of Oxides," The Journal of Australian Institute of Metals, 7[2], pp. 109-114 (1962). 4. G.B. Hoflund, "Characterization Study of Oxidized Polycrystalline Tin Oxide Surfaces before and [4] after Reduction in H2," Chemical Material, 6 , pp. 562-568 (1994). 5. D. Morgan, D.P. Anderson and P. Kim, Rockwell International Science Center, "Solderability As- sessment via Sequential Electrochemical Reduc- tion Analysis," accepted for publication in Journal of Applied Electrochemistry.

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